Estudio de la densidad corneal medida con cámara de scheimpflug en sujetos con queratocono

Abstract

Keratoconus is a bilateral noninflammatory corneal ectasia in wich the cornea assumes a conical shape due to stromal thinning. Aetiology is unknown. Corneal transparency depends on histologic characteristics, specially on the morfology and homogeneity distribution of collagen fibers. It can be measured by intraocular light scattering, an optical phenomenon that degrades the retinal image. It has two elements: forward scattering (towards the retina) and backscattering (towards the light source). The Pentacam is a Scheimpflug camera based system that obtains anterior segments images and permits measuring corneal density or backscattered light. Our hypothesis Is that optical corneal density measured with the Scheimpflug camera is altered in keratoconus, so we analyse if corneal density measured with the Scheimpflug camera at the apex and mid periphery, are different than those in subjects with high astigmatism (> 3 diopters (D)) and general population. As a secondary purpose, 1) the relation between corneal density and the severity of keratoconus is studied. 2) Progression of keratoconus is measured in terms of increasing of corneal astigmatism diopters during a mean period of 34 months (range 11-62 months). 3) Correlation between central corneal density and mid periphery is also analyzed in all groups. 4) Corneal density at differents distances from the center is measured in healthy subjects in order to determine a threshold value and a distribution pattern. 5) The relation between corneal density and age is also analysed. Apex corneal density in keratoconus was higher than at the center of the control group and the astigmatism group (p= p<0,001 y p=0,001 respectively). Mid periphery corneal density was also higher in keratoconus, but no statistic differences were found. Due to corneal transparency is based on its structural characteristics, these results could be explained because of corneal tissue alterations occurring in keratoconus such as colagen fibers, keratocytes or ground substance alterations as are described in previos studies. No statistic differences were found when comparing central and mid periphery corneal density between the astigmatism group and the control group (p=0,819 y p=0,434 respectively). So, astigmatism itself may not be a confounding variable. A positive correlation between the severity of keratoconus and corneal density at the apex was found (r=0,213; p=0,043). According to these results, corneal transparency may be a consequence of keratoconus worsening. Corneal density in keratoconus as a progression parameter was studied. Progression was measured as increasing of astigmatism diopters. Corneal density values were higher in those subjects with more increasing of corneal astigmatism, but no statistic significant differences were found (p=0,216 when augmenting 1 or more diopters; p=0,260 when augmenting 2 or more diopters). A larger sample and a longer monitoring period could obtain statistic significant differences. We found a correlation between apex and mid periphery corneal density in all groups (r=0,465 and p<0,001 for the keratoconus group; r=0,760 and p<0,001 for the astigmatism group; r=0,738 and p<0,001 for the control group). Corneal density at different points was analyzed in order to stablish threshold values and a distribution pattern. Our values were higher than those described in other published studies, but as they do, we also found a decreasing tendency of density values from the apex to the periphery. No significant differences were found in comparing the central corneal density values to some points located at different distances. The correlation between corneal density and age was studied. There were significant differences between the mid periphery corneal density values and age in controls and the control and astigmatism group (r=0,414; p=0,003 and r=0,374; p<0,001, respectively). Conclusions: 1. Apex corneal density in keratoconus measured with Pentacam was higher than at the center of the control group and the astigmatism group. 2. No statistic significant differences were found when comparing mid periphery corneal density between keratoconus and contro or stigmatism groups. 3. No statistic significant differences were found when comparing central and mid periphery corneal density between the astigmatism group and the control group. So, astigmatism itself may not be a confounding variable. 4. A positive correlation between the severity of keratoconus and corneal density at the apex was found. 5. No statistic significant differences were found when comparing corneal density and progression (incrasing of 1 or more astigmatism diopters) in keratoconus. A larger sample and a longer monitoring period could obtain significant differences. 6. A positive correlation between apex or central corneal density and mid periphery corneal density was found in all groups. 7. Corneal density at different points in healthy subjects was higher than previous plublished studies. No statistic significant differences were found when comparing corneal density values at the center and at different distances. 8. Statistic significant differences between the mid periphery corneal density values and age were found in controls and controls and astigmatism group.